The present invention relates to an electron gun for a color cathode ray tube, and more particularly, to an electron gun for a color cathode ray tube for passing an outer electron beam through the center of an outer main lens to thereby prevent distorted halo caused by the one-sided concentration of the electron beam.
As shown in FIG. 1, in a general cathode ray tube, electrodes are placed perpendicular to the path of an electron beam, so that electron beams 13, 14 and 15 generated from three cathodes 3 have a predetermined intensity and collide against a screen 17 via a shadow mask 16 in order to emit light. A control electrode 4, an accelerating electrode 5 spaced apart from the control electrode 4 by a predetermined distance, and first through fourth accelerating/focusing electrodes 6, 7, 8 and 9 are positioned in the cathode ray tube.
When a heater 2 built in the cathode 3 is heated by receiving power from a stem pin 1, thermions emitted from the cathode 3 due to the heat. Then the thermions are controlled by the control electrode 4, and are accelerated by the accelerating electrode 5. The diverging angle of the electron beams is reduced by the first, second and third accelerating/focusing electrodes 6, 7 and 8, which form a pre-focus lens. The electron beams are converged and accelerated by the third and fourth accelerating/focusing electrodes 8 and 9, which form a main lens. Then, the electron beams pass through the shadow mask 16 placed in front of the phosphorous surfaced screen 17 and collide with the screen 17 to emit light.
FIG. 2 shows the structure of a large-diameter main lens for an electron gun. The diameter of the main lens increases in order to reduce spherical aberration for the main lens portion. Track-shaped rims 18 and 19, which are common for the red R, green G and blue B electron beams, are formed on the facing surfaces of third and fourth accelerating/focusing electrodes 8 and 9. Electric-field control electrodes 20 and 21 are installed behind the rims 18 and 19 by a predetermined distance.
The diameter of the main lens formed as in FIG. 3 may be increased by varying the position of electrodes 18 and 19. However, as shown in FIGS. 4A and 4B, the converging degree of outer beams 13" and 15" advancing in the direction of central beam 14 becomes greater than that of the outer beams 13' and 15' advancing in the opposite direction of the central beam 14. For this reason, halos 24 and 25 appear on the screen 17 from the outer electron beams 213 and 215, as shown in FIG. 6A. In FIGS. 4A-5B, the X-axis represents the distance between the cathode and screen and the Y-axis represents the amount of outer beam focused on the center beam.
FIGS. 15A and 15B show patterns of lenses produced by the electron gun. Particularly, FIG. 15A shows a case where the centers of main lenses 220 and 23 are positioned outside of outer beams. FIG. 15B shows a case where the centers of the main lenses 220 and 23 are positioned between the outer beams. In case of FIG. 15B, the converging degree of the outer electron beams and their spot shapes on the screen 17 are shown in FIGS. 4A, 4B and 6A. The case of FIG. 15A has characteristics opposite to the case of FIG. 15B. In FIGS. 15A and 15B, the reference numeral 43 represents conventional triode auxiliary lenses, and the reference numeral 44 represents conventional outer and center pre-focus lenses.
As explained above, in order to reduce spherical aberration, the diameter of the main lenses is increased so that the center of the outer electron beams deviates from that of the outer main lenses. This differentiates the converging degrees of the electron beams passing through the main lenses in the direction of the central electron beam and in the direction opposite thereto. Accordingly, the electron beam on the screen in is shaped so that a halo is formed on one side, which deteriorates the focusing characteristic of the outer beam and the resolution of the screen 17. Further, the conventional electron gun cannot simultaneously satisfy the balanced converging degree of the three electron beams and the difference in the converging degree of the horizontal and vertical electron beams.